This investigation plans to conduct a systematic diagnostic and treatment outcome study of 600 adolescent drug abuse clinets, aged 21 years or less, at pretreatment in nine "drug-free" outpatient treatment programs; and 144 such clients in two inpatient programs, with the following specific aims: (1) To develop a new valid and reliable diagmostic instrument ("Adolescent drug Abuse Diagnosis" [ADAD]) that will be useful for improving client diagnostic services, for evaluation of client treatment needs, for treatment planning, and to enhance the research potential with adolescent drug abuse clients. (Study #1); (2) To develop for distribution to the field an instruction and scoring manual which will enable and facilitate use of the ADAD instrument by clinical and research personnel in programs for adolescent drug abusers. (Study #1); (3) To test the usefulness of this new instrument for predicting outcome of outpatient treatment. (Study #1); (4) To develop a meaningful and useful classification system (typology) of adolescent drug abuse clients. (Study #2); (5) To develop a method for matching types (subgroups) of adolescent drug abuse clients to two types of treatment programs (environments): outpatient and inpatient. (Study #3).